Excerpt from Chapter 4.2:
Let us now illustrate the energy principle with a more complicated problem, the screw jack shown in Fig. 4–5. A handle 20 inches long is used to turn the screw, which has 10 threads to the inch. We would like to know how much force would be needed at the handle to lift one ton (2000 pounds). If we want to lift the ton one inch, say, then we must turn the handle around ten times. When it goes around once it goes approximately 126 inches. The handle must thus travel 1260 inches, and if we used various pulleys, etc., we would be lifting our one ton with an unknown smaller weight W applied to the end of the handle. So we find out that W is about 1.6 pounds. This is a result of the conservation of energy.
I understand Feynman mathematically, 1.6 pounds * 1260 inches ~ 2000 pounds 1 inch up, but I don't understand his explanation visually.
- Why does the handle go 126 inches when it traverses the jack once?
- How exactly would we use the pulleys and the weight $W$? It's hard for me to picture this. I can only imagine a person pushing the handle with their hands.